1. Field of the Invention
This invention relates generally to non-destructive inspection and, more particularly, to dimensional inspection of fabricated components. In particular, inspection and measurement of complex shapes having substantially all spline surfaces and therefore no available reference datum.
2. Description of the Related Art
Dimensional inspection techniques are used in many applications where the non-destructive evaluation (NDE) of a work piece or component is desired. Known inspection techniques include a visual or manual inspection to facilitate determining a service condition of a component. A knowledgeable and skilled technician may be able to ascertain the worthiness of a particular component for replacement using visual or manual inspection, however visual or manual inspection may not be accurate enough for all components nor repeatable for quality verification purposes. Accordingly, many industrial applications require accurate and rapid measurement of the 3-D shapes of objects. Representative applications of 3-D shape measurement include reverse engineering, 3-D replication, inspection and quality control. In most of these applications, users need to construct 3-D point clouds that correspond to the object's surface by performing measurement on the object's surfaces. Manufacturing industry needs a fast inspection process that can measure and analyze various 3-D features on the part and determine if a feature is within the tolerance specifications or not. The measurement scheme needs to be adequately accurate to eliminate measurement errors. Measurement errors can lead to erroneous inspection that results in an acceptable part being rejected and a defective part being accepted. Hence, both inspection speed and accuracy are equally important.
In particular, precision manufacturing has always required a great deal of dimensional accuracy. Particularly challenging to the precision manufacturing industry is the need to measure complex shapes. Traditionally, complex shapes had to be measured by physical contact using coordinate measurement machines, or CMMs. In order to obtain the requisite dimensional data, CMMs move tiny contact probes around the surface of the object being measured and build up a set of measurement points that are used to ensure that the object measured is within certain dimensional tolerances. However, the measuring process is slow and cannot be used for real-time control. Moreover, when a large number of points are needed to characterize a surface, the data-acquisition time becomes prohibitive since with CMM data acquisition is limited to a few points per second. Moreover, CMMs do not permit in-process measurement of surfaces which would let machine operators measure a surface before a process and make any necessary changes before completing the entire manufacturing loop.
On the other hand, optical-based techniques are usually very fast. Therefore, a possible way to perform the 3-D inspection is to use digital cameras to construct a dense point cloud (e.g., points spaced less then 0.25 mm apart) corresponding to the part being inspected and then analyze the point cloud to determine if it meets the tolerance specifications. But accuracy associated with the conventional camera based inspection techniques has not been very high in the area of measurement of geometrically complex 3-D shapes. Unfortunately, however, in those cases where the shape has substantially all spline surfaces, there are effectively no regular surface or edges that can successfully be used as a reference datum. Therefore, since a reference datum is not available, relative measurements where one compares a measured shape to a reference (ideal) are difficult if not impossible.
Therefore, improved techniques for measuring three dimensional complex shapes where no reference datum is available is required.